The following is a description of a general frame structure used in a wireless access system.
FIG. 1 illustrates a frame structure used in a broadband wireless access system (for example, IEEE 802.16).
A horizontal axis of a frame structure shown in FIG. 1 represents an Orthogonal Frequency Division Multiple Access (OFDMA) symbol number and a vertical axis represents a subchannel (i.e., frequency unit) logical number. In FIG. 1, one frame is defined as a data sequence channel having a predetermined period of time in terms of physical characteristics. That is, one frame is divided into one downlink subframe and one uplink subframe. The downlink subframe and the uplink subframe are separated by a Transmit Transition Gap (TTG) and frames are separated by a Receive Transition Gap (RTG).
Here, the downlink subframe may include a preamble, a Frame Control Header (FCH), a downlink MAP (DL-MAP), an uplink MAP (UL-MAP), and one or more data bursts. The uplink subframe may include one or more uplink data bursts and a ranging subchannel.
In FIG. 1, the preamble is specific sequence data located at a first symbol of each frame and is used for synchronization of a terminal or Mobile Station (MS) with a base station or for channel estimation. The FCH is used to provide DL-MAP-related channel allocation information and channel code information. The DL-MAP/UL-MAP is a Medium Access Control (MAC) message used to notify a terminal of channel resource allocation in uplink/downlink. The data burst is a unit of data for transmission from a base station to a terminal or from a terminal to a base station.
A Downlink Channel Descriptor (DCD) that can be used in the frame structure of FIG. 1 is a MAC message used for notification of physical characteristics in a downlink channel and an Uplink Channel Descriptor (UCD) is a MAC message used for notification of physical characteristics in an uplink channel.
In the case of downlink, the terminal detects a preamble transmitted from the base station to achieve synchronization with the base station as shown in FIG. 1. Then, the terminal can decode the DL-MAP using information obtained from the FCH. The base station can transmit scheduling information for downlink or uplink resource allocation to the terminal every frame (for example, 5 ms) using the DL-MAP or UL-MAP message.
The following is a description of a general data transmission method for a transmitting end and a receiving end.
In the data transmission method, when the receiving end has failed to receive data transmitted from the transmitting end, the receiving end requests that the transmitting end retransmit the same data. One general data retransmission method is an Automatic Repeat reQuest (ARQ) scheme.
In the ARQ scheme, after the receiving end receives data, the receiving end notifies the transmitting end of whether or not the receiving end has successfully received the data through an Acknowledgement (ACK) or Non-Acknowledgement (NACK) signal and the transmitting end retransmits corresponding data when a NACK signal is received. The ARQ scheme is divided into Stop-And-Wait (SAW) ARQ, Go-Back-N (GBN) ARQ, Selective-Repeat (SR) ARQ schemes.
In the case where the SAW ARQ scheme is used, the transmitting end waits until an ACK or NACK signal is received after transmitting data. The transmitting end transmits next data when receiving an ACK signal and transmits previous data when receiving a NACK signal. That is, in the SAW ARQ scheme in which the transmitting end transmits one frame at once, the transmitting end transmits a next frame after confirming that a frame has been successfully transmitted.
In the GBN ARQ scheme, the transmitting end continually transmits data, regardless of a response message. Specifically, in the case where the receiving end has failed to receive data of a specific frame, the receiving end does not transmit an ACK signal of the specific frame to the transmitting end. Since the transmitting end fails to receive an ACK signal of the specific frame, the transmitting end retransmits data, starting from data of the specific frame.
In the SR ARQ scheme, when the transmitting end has received a NACK signal while continually transmitting data, the transmitting end retransmits only data corresponding to the NACK signal. Specifically, when the receiving end has failed to receive data of a specific frame, the receiving end transmits a NACK signal to the transmitting end. When the transmitting end has received a NACK signal, the transmitting end retransmits data of a frame indicated by the NACK signal to the receiving end, thereby transmitting all data. Implementation of the SR ARQ scheme may be relatively complex since each frame should be individually managed by assigning a sequence number to each frame.
The data transmission rate required in the scheme in which data is transmitted in a packet format has increased as communication technologies have improved. Accordingly, a coding rate or a modulation method of a level corresponding to the high data rate has also been applied to the communication system in order to prevent errors occurring in high-speed transmission environments. In addition, there has been a need to provide an ARQ scheme suitable for high-speed transmission environments. Thus, a Hybrid ARQ (HARQ) scheme has been suggested.
In the ARQ scheme, information is discarded when an error occurs in the information. However, in the HARQ scheme, the receiving end stores information in which an error has occurred in a buffer and then combines the stored information with information for retransmission to apply Forward Error Correction (FEC). That is, the HARQ scheme can be considered a combination of the ARQ scheme with FEC. HARQ can be mainly divided into the following four types.
In the first type of HARQ scheme, the receiving end always checks an error detection code included in data to preferentially apply the FEC scheme. When a received packet contains an error, the receiving end requests retransmission of the received packet to the transmitting end. The receiving end discards the erroneous packet and the transmitting end uses the same FEC code as that of the discarded packet to retransmit the packet.
The second type of HARQ scheme is referred to as an “Incremental Redundancy (IR) ARQ scheme”. In the second type of HARQ scheme, the receiving end stores an initially transmitted packet in a buffer without discarding the packet and then combines the stored packet with retransmitted redundancy bits. During retransmission, the transmitting end retransmits only parity bits excluding data bits. The transmitting end uses different parity bits every retransmission.
The third type of HARQ scheme is a special case of the second type. Each packet is self-decodable. When the transmitting end performs retransmission, the transmitting end retransmits constructs and retransmits a packet including both data and an erroneous portion. Although this scheme enables more correct decoding than the second type, the efficiency of coding gain is low.
The fourth type of HARQ scheme provides a function to store data initially received by the receiving end and combine the stored data with retransmitted data in addition to the functions of the first type. The fourth type of HARQ scheme is also referred to as “metric combining” or “chase combining”. The fourth HARQ type has an excellent Signal to Interference Noise Ratio (SINR) and always uses the same parity bits of data to be retransmitted.
When an error has occurred or data has been lost during data transmission, the above data retransmission methods enable reconstruction of original data using the above schemes.
FIG. 2 illustrates example HARQ control signal delay during general downlink data transmission.
As shown in FIG. 2, a base station can notify a terminal of downlink burst information of the current frame by sending a DL-MAP to the terminal in a frame (for example, an Nth frame) in a broadband wireless access system (for example, WiMAX). Accordingly, the terminal can receive downlink data burst from the base station in the Nth frame (S0101).
In addition, the base station can notify the terminal of information of an uplink channel for transmitting a control signal (for example, an ACK signal) by sending a UL-MAP to the terminal in the Nth frame. Accordingly, when HARQ is applied, the terminal can transmit an ACK/NACK signal for the downlink data burst to the base station in the N+1th frame (S0102).